When loop devices are re-used, the disk sequence number is increased.
Parse it when creating a loop device and store it.
The kernel will never return DISKSEQ=0, so use it to signal that it's
not supported by the current kernel.
LOOP_CONFIGURE allows us to configure a loopback device in one ioctl
instead of two, which is not just faster but also removes the race that
udev might start probing the device before we adjusted things properly.
Unfortunately LOOP_CONFIGURE is broken in regards to LO_FLAGS_PARTSCAN
as of kernel 5.8.0. This patch contains a work-around for that, to
fallback to old behaviour if partition scanning is requested but does
not work. Sucks a bit.
Proposed upstream fix for that issue:
https://lkml.org/lkml/2020/8/6/97
